The invention is directed generally to web transports, and particularly to a system for controlling the tension in a web as it is transported from a supply spool to a take-up spool.
In systems which transport web material such as microfilm at high rates of speed, a lack of proper tension in the web allows air to become trapped between adjacent layers of web on the take-up spool. As a result, sliding or "cinching" frequently occurs between adjacent layers of web on the take-up spool.
In an attempt to avoid such cinching, some prior web transports have applied a constant hold back torque on the supply spool. Tension is thus imposed on the web and, if the radii of the spools do not change appreciably as the web is collected on the take-up spool, a fairly constant tension is imposed on the web and little cinching occurs.
In systems in which larger amounts of web are transported, the radii of the take-up and supply spools may change by a factor of two or more as the web is advanced from the supply spool to the take-up spool. Because web tension varies in inverse proportion to spool radius, at least where a constant hold back torque is applied to the supply spool, the web tension varies considerably. Consequently, it is difficult to avoid cinching and simultaneously avoid imposing too much tension on the web.
Other transport systems employ a capstan between the take-up and supply spools to control web velocity, and apply a constant torque on the motor which rotates the take-up spool. Once again, web tension varies as the spool's radii changes. This problem is commonly corrected by mechanically sensing web tension and servo-controlling the torque on the take-up motor. However, such methods of controlling web tension are too expensive for many applications.
Accordingly, a general object of the invention is to provide an improved system for controlling web tension in a web transport system.
It is a more specific object of the invention to provide a reliable and inexpensive system for holding web tension substantially constant, irrespective of large changes in the radii of the supply and take-up spools.